Spinels are well known minerals having the generic structure M.sup.I M.sub.2.sup.II O.sub.4 wherein M.sup.I represents one or more metal atoms having a valence such that when chemically bound with metal atoms of M.sup.II, which may be one or more metal atoms the same as or different from M.sup.I but having a different valence, exhibit a total combined valence taking into account the number of atoms in the formula as to equal eight (8). In the ruby spinel iron, magnesium and aluminum combine in the generic formula as Fe.sup.++ Mg.sup.++ (Fe.sup.+++ Al.sup.+++).sub.2 O.sub.4. Man made spinels have also been made such as for example Co.sup.++ (Co.sup.+++).sub.2 O.sub.4, Co.sup.++ (Al.sup.+++).sub.2 O.sub.4, MgAl.sub.2 O.sub.4, (NiCo)Al.sub.2 O.sub.4, (MgCo)Al.sub.2 O.sub.4 and the like.
Spinels disclosed in the prior art have been prepared by admixing the desired metals, as their oxides, in the theoretical proportions and heating them to temperatures above about 1500.degree. C. where at no amount of continued heating at this temperature nor heating to a moderately higher temperature materially changes the density or crystalline structure. Of course extreme continued higher heating can change the crystalline structure to a structure not associated with the spinel structure. When spinels have been prepared in this manner the usual utility requires the spinel to be ground into a fine powder, mixed with a binder, shaped, and the binder burned out. Such techniques have produced shapes which have a degree of porosity about that of any conventional construction using a binder to maintain such shape during firing.
Most prior art techniques used commercially for preparing ceramic spinels employ the fusion technique of the metal oxides. This technique is not wholly satisfactory for the preparation of ceramic spinels because the metal atoms may not completely form into the spinel lattice structure due to poor batch stoichiometry; that is, some metal atoms form a segregated oxide phase admixed with the spinel lattice structure and once formed by fusion the crystals are not amenable to shaping by pressure and sintering without aid of binders which may be detrimental to acid and/or base resistance and physical properties of the finished product. Organic binders in ceramics made in this way make the body relatively porous when they are removed during or after shaping. Segregated ceramic binders may weaken the body because they are the site of differential expansion and contraction and/or chemical attack. The prior art also recognized the phenomena of spinel formation being a physio-chemical reaction based on thermal conditions such that, regardless of the ratio of the metals, some spinel lattice would form at the correct temperature, physical and chemical conditions, albeit those atoms not forming a spinel lattice structure remain as segregated phases of the metal oxides. The spinel shapes commercially available usually have been prepared from spinels produced from starting materials containing impurities or one or more segregated metal oxides phases and thus are relatively poor with respect to their physical properties, e.g., tensile strength, acid and/or base resistance and porosity.
Numerous patents and scientific literature have been published disclosing different techniques for preparing spinels (esp. MgAl.sub.2 O.sub.4). Most procedures employ metal oxides or oxidizable compounds, both of which are converted to a spinel by firing or fusion with or without pressure.
In some patents a magnesium compound and an aluminum compound are mixed to give the requisite molecular constitution, wet ground and mixed, and fired at temperatures up to 3,000.degree. F. (ca 1660.degree. C.) as for example, in U.S. Pat. No. 2,618,566 or shaped before firing into pebbles as in U.S. Pat. No. 2,805,167.
Others use pure magnesia and alumina mixtures which are then fired at 2150.degree. C. and cooled slowly overnight, (e.g. U.S. Pat. No. 3,516,839). Still others mix alumina with magnesium nitrate, dry fire on a schedule to 1400.degree. C., and then ground to obtain a powder, (e.g. U.S. Pat. No. 3,530,209). Another technique follows the fusion route of magnesium nitrate hexahydrate and ammonium aluminum sulfate dodecahydrate (both reagent grade) to 1300.degree. C. to produce a fine powder, (e.g. U.S. Pat. No. 3,531,308). A magnesium-salt (MgSO.sub.4.7H.sub.2 O), aluminum-salt (Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O) mixture, co-crystal has been employed to prepare a powder which is then shaped into ceramic bodies by hot press techniques with or without the use of binders, (e.g. U.S. Pat. No. 3,544,266).
Concomitant with these developments researchers investigated the nature of metal double hydroxides formed by coprecipitation, some of which were shown to convert to a spinel upon calcination. Early work was performed by Feitnecht and his students who made a series of double hydroxides with Mg/Al ratios between 1.5 and 4 to 1, respectively, by coprecipitation from magnesium and aluminum chlorides, Helv. Chim Acta 25, 106-31 (1942), 27, 1495-1501 (1944). No change could be detected by X-ray diffraction techniques then available for different Mg/Al ratios or a certain degree of substitution by chloride for hydroxide. A similar double hydroxide, reported to be a hydrate even after heating to 150.degree. C., was reported by Cole and Hueber in "Silicates Industriels" Vol. 11, pp 75-85 (1957). The compound was made by the reaction of NaOH with Al metal or Al.sub.2 (SO.sub.4).sub.3 and MgO or MgSO.sub.4 at 65.degree.-70.degree. C. The product had a Mg/Al ratio of 4/1 even when reactant proportions were varied. However, Mg(OH).sub.2 was observed as a second phase in some cases.
More recently, Bratton in both Journal of The American Ceramic Society, Vol. 52, No. 8 (2969), and Ceramic Bulletin, 48, #8 pp 759-62 (1969) 48, 11, pp 1569-75, reported the coprecipitation of numerous magnesium and aluminium chlorides and oxalates which on heating, drying, calcining or firing, exhibited a spinel X-ray diffraction crystallographic pattern. The coprecipitation product resulted in a magnesium aluminum double hydroxide of composition 2Mg(OH).sub.2. Al(OH).sub.3, plus a large amount of segregated gibbsite Al(OH).sub.3 phase (see also U.S. Pat. No. 3,567,472). This is presumably the same product Feitnecht obtained.
Bakker and Lindsay in "Ceramic Bulletin" Vol. 46, No. 11, pp 1095-1097 (1967) report that a high density spinel body can be made from Mg(OH).sub.2 and Al(OH).sub.3 if 1.5% AlF.sub.3 is added as a mineralizer.
In the works cited above these powders were, in some instances, calcined then fired while in other instances the powders were heated through the calcining range and ultimately through the firing and even the fusion range. Early work was directed to preparing spinel usable as a decolorant, U.S. Pat. Nos. 2,395,931 and 2,413,184 or antacids, U.S. Pat. Nos. 3,323,992 and 3,300,277. In the last case a "highly hydrated magnesium aluminate" is claimed as a new composition of matter, the formula of which is Mg(OH).sub.2.2Al(OH).sub.3. XH.sub.2 O where X=4 to 8. The material is prepared by the reaction of NaAlO.sub.2 (Na.sub.2 Al.sub.2 O.sub.4), NaOH and MgCl.sub.2 as aqueous solutions at a pH from 8-9. Bratton in U.S. Pat. No. 3,567,472 also discloses coprecipitation of a magnesium and aluminium chloride from a solution having a pH from 9.5 to 10, drying or firing to obtain a light-transmitting spinel by adding CaO.
While the pressure molding technique is commercially desirable for large shapes such as fire brick and the like, it would be advantageous to be able to slip-cast the spinel powders for use in making intricate shapes such as furnace ware, crucibles and the like. Previous attempts to slip-cast the spinel after firing the powders of course met with failure as did the first attempts to slip cast the precursor calcined spinel powders of our co-workers.